The present invention relates to improvements in an exhaust emission control system for an internal combustion engine, more particularly to an exhaust emission control system provided with a NOx storage type three-way catalyst.
A Japanese Patent Provisional Publication No. 6-129246 discloses an exhaust emission control apparatus which is arranged to store NOx when an engine is operated on an air-fuel mixture having a air-fuel ratio leaner than a stoichiometric ratio (lean operation) and to release the stored NOx when the air-fuel ratio of the mixture becomes richer than the stoichiometric ratio (rich operation). Further, the conventional apparatus is arranged to periodically execute a rich-spike treatment that temporally varies the air-fuel ratio of the mixture into a richer ratio as compared with the stoichiometric ratio to release the stored NOx and to purify the released NOx by means of HC and CO, which increases in volume during the rich operation and function as a reduction agent.